1. Field of the Invention
The present invention relates to a process and apparatus for forming inorganic glassy mixtures, in part from toxic wastes, by the process called vitrification, so that the toxic wastes become an integral part of the vitrified glasses. The invention also provides a method of encapsulating high level nuclear wastes and other toxic wastes by vitrification.
2. Description of the Prior Art
Glass melters have been developed in the U.S., Europe, and Japan for the conversion of high level radioactive waste (HLW) to borosilicate glass for disposal. Laboratory and pilot scale operations have been conducted to develop equipment, glass compositions, and control methods. The melters developed fall into four categories: batch melters, continuous pot melters, Joule-heated ceramic-lined melters, and stirred melters. The newest design, the stirred melter, combines the higher production rates and higher glass quality features of the joule-heated melters with the lower-cost, compact simple maintenance of the pot melters.
The first waste glass melters were designed for batch operations, and were a direct increase in scale from crucible tests. This approach was found unsuitable for HLW production facilities because of slow melt rates caused by slow heat transfer from the external heaters through the canister into the reacting batch. Lack of agitation and lack of temperature uniformity made it difficult to homogenize the glass. Calcination of feed before introduction into the canister increased the melt rate, but also increased the tendency for crystal formation in the glass and entrained waste in the calciner off-gas system. This method was finally eliminated for HLW based on the large number of melters, operating in parallel, required to meet the production rates necessary to dispose of HLW inventories. The method remains attractive for small facilities where melt rates are not restrictive, such as the on-line verification of certain radioactive wastes. Subsequent improvements in melter materials and glass compositions permit the slow production of small amounts of waste glass with satisfactory durabilities.
The second class of melters developed consisted of continuous pot melters. In this type the melt rate was increased by increasing the diameter of the pot, by direct heating of the pot by radio frequency induction heating, and by continuous feeding of raw materials. Glass homogeneity was improved by using gas bubblers to agitate the melt. The largest of this type of melter is the French Atelier de Vitrification de Mercoule (AVM) system which melts about 25 kilograms per hour. This is the processing rate limit per pot melter using dried feed. With slurry feeding the melter capacity limit would be about one half of this, or 12 kilograms per hour per melter. The melter design temperature is limited to about 1150.degree. C. by creep resistance of the Inconel.RTM. alloy used for the pot. The use of this system was not practical because of the large number of parallel melters, calciners, and off-gas scrubbing systems required. However, this class of melter is modularized, with parts that are relatively easy to replace. An additional benefit of this approach is that only the failed components need be replaced, minimizing the amount of waste generated with melter changeout, and maximizing the useful life of each component. This approach therefore has merit when dealing with homogeneous wastes, and where melt rate restrictions are not limiting. For non-homogeneous wastes the close coupling of the calciner/incinerator functions with the melter is not desirable, since it is necessary to classify the calcined waste to remove large metallic pieces.
A third category of melters consists of the joule-heated, ceramic-lined melters. This is the result of several generations of melter development based originally on commercial, electrically heated melters. Major differences between HLW melters and ceramic-lined melters have been:
the use of metal shells to contain glass contact refractories and thermal insulation; PA1 the development of specialized slurry feeding and glass pouring systems; and PA1 the use of nickel based alloys for electrodes carefully matched with glass composition control. PA1 1) It is constructed of high-temperature metal alloys, such as Inconel.RTM. 690, and is capable of operating with the higher glass melt viscosities existing at temperatures of 1075.degree. C. or lower, such as 700-900.degree. C. PA1 2) It is designed with narrow channels of minimal height in order to minimize glass melt hold-up in the extruder and possessing the good mixing characteristics of a mixing screw in a single screw extruder or the good mixing characteristics of a twin screw extruder. Maximum channel height suitable for this invention is about one tenth the diameter of the extruder screw in order to produce the high shear rates in the metering section that are required for the invention. These features also result in high throughputs and low residence times. Thus, production rates per unit of equipment size are many times greater than in conventional ceramic glass melters. PA1 3) It is designed as a multi-stage extruder, or with two or more extruders in series, so that all of the processes involved in glass melting and its homogenization can take place in the extruder; e.g., drying in a first stage, gas removal in a second stage, and glass melt homogenization in a third stage. PA1 4) To the extent desired and necessary to supplement heat generation by viscous dissipation, it is designed for induction, radiant or other indirect heating, rather than for heating by means of electrodes. This is made possible by the modular design features of extruders, and by their suitability for operation at lower temperatures and higher viscosities than prior-art vitrification equipment, so that at least a portion of the needed heat input is provided by viscous dissipation, i.e., internal friction in the glass melt. PA1 5) It is designed for operation under pressure to minimize loss of radioactive gases and vapors, which is not possible in conventional ceramic glass melters. PA1 6) It is designed with several feed ports along its length to make it possible to adjust glass composition for maximum melting rates. The screw or screws are preferably arranged to generate lower pressures at the ports. PA1 7) It is designed with the barrel or barrels having many sections held together with special fasteners to facilitate robotic disassembly and maintenance operations.
This type of melter is lined with refractory, and the glass is directly heated by conducting electricity through the melt. This system with slurry feeding has been selected for all the production melter systems in the U.S., W. Germany, and Japan because of the higher production rates and high glass quality. The size of these systems is effectively limited only by operating facility constraints (e.g., cell space, crane capacity), since all the structural support is provided by a room temperature metal box which contains the refractory. The Inconel.RTM. 690 alloy electrodes only need to be self-supporting, and high current densities are possible on the faces of the electrodes. Therefore, nominal melt temperatures can be as high as 1150.degree. C., which is only 200.degree. C. lower than the alloy melting point. Glass production rates are proportional to the surface area of the melt, but convection caused by the joule heating is enhanced as the size of the melter is increased, so larger melters have proportionately higher melt rates. Small laboratory melters operate below 22 kilograms per hour per square meter, whereas production melters operate at about 39 kilograms per hour per square meter. Melt rates can be doubled by dry feeding. The combination of higher temperature and convective mixing makes the glass homogeneous. The major difficulty in slurry-fed ceramic-lined designs is the large number of individual refractory bricks, supporting shell and other components that must be assembled to make this type of melter. This complexity increases the melter construction, installation, and disposal costs. In radioactive service only limited repair is possible, so failure of individual components can require removal and disposal of the entire assembly.
A fourth class of melters, known as Advanced or Stirred Melters, has been developed, evolving out of various melt reaction studies, computerized staged reaction models, organic combustion models and melter redox models generated in support of the glass melter in the Defense Waste Processing Facility (DWPF) constructed at the Savannah River Plant in Aiken, S.C. Evaluation of the melt reaction sequence indicated that the melter operating temperature need not be as high as the nominal 1150.degree. C. to assure adequate glass durability. This made possible the consideration of melter designs where Inconel.RTM. 690 components carry dynamic loads, which is not possible with the present nominal operating temperature of 1150.degree. C. Several melter design companies were involved in determining what commercial technology might be applicable. The most promising of these was a proposal by Associated Technical Consultants to develop mechanically stirred melters comparable to those originally investigated by Owens-Illinois, Inc., of Toledo, Ohio. This approach offered the possibility of combining the larger size of the continuous pot melters with the higher production rates of mechanically stirred melters. Owens-Illinois, Inc., demonstrated a compact 0.57 cubic meter melt chamber that produced 10,900 kilograms per day of partially melted commercial glass from raw materials. It featured a simple geometry, with a simple mechanical drive system, plus rapid start, drain and restart capabilities, all of which are desirable properties for radioactive service.
One version of conventional high level radioactive waste vitrification involves introducing the feed slurry from the top of the melter and forming a cold cap on the melt surface as the water evaporates and is removed by the off-gas system. Three electrodes supply energy directly to the melt. The cold cap melts from the bottom and forms the borosilicate waste glass. Molten glass flows from the bottom of the melter up through a riser and falls into a stainless steel canister by either periodic batch pouring or continuously. Glass pouring is activated by airlifting.
As described above, a slurry feed enters the melter and forms a feedpile or cold cap on the surface of the glass pool which is composed of two parts--a boiling slurry layer where water is driven out of the feed, and an underlying crust layer where decomposition of the waste chemicals and glass formers occurs to produce a borosilicate glass. Newly formed glass sloughs off the bottom of the feedpile, enters and mixes with the glass already in the glass pool at 1150.degree. C., and is periodically poured into a stainless steel canister. Here the molten glass cools and solidifies to form the final product of radioactive waste glass.
Conventional glass melters for vitrification of nuclear wastes depend upon natural convection in the glass pool to provide reasonably good mixing of fresh glass from the feedpile with glass in the melt pool. Thus, a retention time of some 40 to 80 hours in the melter is required to complete this operation.
One of the waste acceptance specifications is that the chemical composition of the waste glass must be specified for all elements, except oxygen, which are present in concentrations greater than 0.5 weight percent. Various processing requirements must also be satisfied. For example, the slurry must be pumpable between tanks and it must form a homogeneous mixture. In the melter, the glass must be in a specified viscosity range so that it mixes well and can be poured into a canister. The oxidation state of various chemical species in the glass must also be controlled to prevent excessive foaming and possible precipitation of metals on the bottom of the glass pool. Finally, the glass composition must fall within the envelope of acceptable glass compositions which show good durability as measured by the leach test. The above requirements on chemical composition, processibility, and durability make it imperative that the vitrification process be well controlled.
One test as a measure of the quality of the glass incorporating the nuclear wastes is a static leach test in deionized water at 90.degree. C. for 28 days. The glass must have a release rate less than 1 gram per square meter per day for Na, Si, B, Cs-137, and U-238. This requirement includes two elements and two radionuclides that have to be measured and reported. Generally, Na and Si are measured in leachates and U-238 can be inferred from the elemental U release rate, which is also generally measured during testing. Measuring Cs-137 during testing is difficult because of the radiation dose rate associated with it.
U.S. Pat. No. 4,778,626 teaches how a dry, pourable particulate mixture of nuclear wastes and synthetic rock-forming components can be produced. However, the nuclear wastes have higher leaching rates than in conventional borosilicate glass vitrification. This patent also describes how a dry nuclear waste can be prepared.
U.S. Pat. No. 4,855,082 teaches how common silica glasses can be used to encapsulate dangerous waste materials. Such silica glasses have melting points of about 800.degree. C., compared to the 1150-1400.degree. C. of the special borosilicate glasses used in conventional vitrification of high level nuclear wastes, whereby heating costs are lower and corrosivity of the glass is much less. This process has two serious disadvantages, namely: (a) that the leaching rates of the nuclear wastes are some 4-fold greater than for the special borosilicate glasses; and (b) that a portion of the nuclear wastes is not an integral part of the glass itself.
FIG. 2 of U.S. Pat. No. 4,855,082 shows a helical blade on a shaft. The helical blade, which defines a uniform deep channel, is used to mix a solid waste with molten glass. The solid waste and the molten glass are prepared separately prior to being fed into the apparatus. The two feed materials are mixed by radial blades near the outlet of the apparatus, partially cooled and extruded from the apparatus as a strand. Due to the deep channel defined by the helical blade, the helical blade acts much like a "solid conveyor" and produces negligible shear rates. U.S. Pat. No. 4,855,082 does not contemplate formation of a glass melt within the disclosed apparatus, which is not capable of forming a glass melt from components of toxic wastes to supplement a glass frit feedstock.
The present invention is directed at providing a process and apparatus for overcoming the problems of prior-art melters by exploiting the high pressures and high shear rates produced by extruders for the purpose of implementing a relatively-low temperature continuous vitrification process. No prior-art equipment used for vitrification is able to melt a "thermoplastic" glass frit by producing sufficiently high pressures and corresponding high shear rates because such shear rates can be generated only by very shallow channels, such as produced in the metering section of an extruder having channels generally less than one tenth of the diameter of the extruder screw. The helical blades used in prior-art mixing vessels are unable to generate such high pressures and high shear rates because the generation of high pressure by compression requires a decreasing channel depth and the production of high shear rates requires a shallow channel depth, both of which are characteristics of extruders. Thus, the helical blades typically used in prior-art vitrification equipment cannot move glass melt mixes in the 700-900.degree. C. temperature range due to their excessively high viscosities in this range. This can be done only by generating sufficiently high shear rates.
As well understood in the art of extrusion, very viscous liquid materials can barely be poured from a container by gravity. Hence, when such materials are placed under high shear rates (wherein one layer is moved rapidly away from its adjacent layer), internal friction is generated within the viscous liquid, which in turn generates heat. Thus, the energy of the electric motor turning the extruder screw under high torque is converted into frictional heat, which causes the viscous liquid to rise in temperature with a resulting lower viscosity. These extruder features make this invention possible.